1. Field of the Invention
The invention relates to a screw extruder and gear pump arrangement for highly viscous media.
2. Description of the Prior Art
Published German Patent Application DE-A 21 00 403 discloses a hydraulic pump which, in the form of an epicyclic gearbox, is equipped with a sun gear, a plurality of planet gears rotatably mounted on a planet carrier and an internal gear, all the gears having teeth. Between the planet gears and the internal gear, which intermesh, and also between the planet gears and the sun gear, which likewise intermesh, suction and pressure chamber elements are formed alternately, being sealed off by arcuate dividing walls of the planet carrier, which otherwise fills the interspaces between the gears. The flat sides of the planet gears, of the internal gear and of the sun gear in each case bear in a leakproof manner on two flat casing walls arranged at a distance from each other, which are provided with wall apertures which connect the individual suction and pressure chamber elements between the gears with a suction chamber provided with a hydraulic feed line and functioning as a collector and, respectively, with a pressure chamber provided with a hydraulic line and likewise functioning as a collector.
Gear pumps of this type, i.e., a planet gear pump, the planet carrier is preferably fixed, while the sun gear and the internal gear revolve. Such a pump is basically unsuitable for delivering highly viscous media, since it needs to be fed with a considerable pilot pressure and thus would not be self cleaning.
A similar gear pump, which is disclosed by DE 36 31 527 C1, permits its delivery rate to be varied independently of speed. The pump again has toothed planet gears, a planet carrier and a toothed internal gear and also a toothed sun gear, which are housed in a pump casing. In order to change the delivery rate, independently of the speed of the driven sun gear, optionally the revolving movement of the planet carrier or the rotational movement of the internal gear can be stopped. The planet carrier is provided with an inlet sealing wall having a plurality of inlet openings and an outlet sealing wall spaced apart axially therefrom and having a plurality of outlet openings. The sealing walls are mounted in a rotatable and sealed manner in the cylindrical pump casing. A suction chamber is arranged in the pump casing upstream of the inlet sealing wall in the delivery direction, and a pressure chamber is arranged downstream of the inlet sealing wall in the delivery direction. The suction chamber in each case extends through the inlet openings into individual suction chamber elements, that extend to the outlet sealing wall, while the pressure chamber extends through the outlet openings into individual pressure chamber elements that extend to the inlet sealing wall.
The suction chamber elements and the pressure chamber elements are arranged in the interspaces between a planet gear, the internal gear and the sun gear and are sealed off from one another by the inlet and the outlet sealing wall and by parts, filling the interspaces, of the solidly designed planet carrier arranged between the inlet and the outlet sealing wall. The feed line for the medium to be delivered runs in the form of a longitudinal bore, which changes into a transverse bore within the axis of rotation of the planet carrier led through the wall of the pump casing, while the discharge line for the delivered medium is guided in a corresponding way through the axis of rotation of the sun gear. Because of the small diameters of the bores and the small cross-sectional areas of the inlet and outlet openings, such a pump would be unsuitable for delivering highly viscous media and would not be self cleaning.
Another gear pump operating as a planet gear pump is disclosed by U.S. Pat. No. 3,870,437. In this case, the column to be delivered is in each case enclosed in the interspace between the sun gear and the internal gear and two immediately adjacent planet gears. In order to enlarge the size of this interspace continuously in the region of the suction point (inflow region) during the revolution of the planet gears in order to produce the pumping action, and in order to reduce it continuously in the region of the pressure zone (outflow region), this pump has flexibly deformable gears. For example, the internal gear and the driven sun gear can be arranged eccentrically in relation to one another, and flexible planet gears can be used, which are able to adapt to the different gap widths between internal gear and sun gear by deformation (ovality).
A common feature of these known planet gear pumps is that not only the planet gears but also the internal gear and the sun gear in each case have teeth, which intermesh with one another with the effect of a revolving gear mechanism.
When processing plastic melts or rubber mixtures, gear pumps have hitherto been usual, operating with the effect of a spur gear mechanism and generally being connected to a screw extruder. As a rule, such gear pumps have a drive which is separate from the drive of the extruder shaft or shafts. Such an extruder is disclosed by EP 0 508 080 A2, for example. Thus, an extruder screw delivers the material to be extruded directly into the pocket area of the gear pump, which has two intermeshing spur gears. This type of gear pump is not self cleaning. Therefore, when the respective material to be processed is changed, such pumps require complicated dismantling and cleaning work in order not to accept any quality losses following the material change.
EP 0 564 884 A1 discloses a double screw extruder whose screw shafts in a section to increase the pressure of the melt to be processed are in each case fitted with one of the two spur gears of a gear pump, so that the gears are driven directly by the screw shafts. By means of two sealing walls at the end, that is to say running at right angles to the longitudinal axis of the screw shafts, which are provided with a passage window for the melt to the inlet side and to the outlet side, suction chamber and pressure chamber are divided from each other. This gear pump is not self cleaning either. Its delivery volume cannot be changed independently of the rotational speed of the screw shafts.
Finally, EP 0 642 913 A1 discloses a single-screw extruder whichhas a gear pump in a spur gear mechanism design prior to its last screw section. One of the two spur gears of the gear pump is fixed directly to the screw shaft of the screw extruder and is driven by said shaft. The extruder casing has a lateral bulge, in which the second spur gear of the gear pump is mounted. In each case a sealing wall bears on the flat sides of the spur gears, being mounted with a form fit in the casing of the extruder. The sealing walls or the extruder casing are in each case provided with a passage window on the suction side and pressure side for the extruded material. No self cleaning is ensured in this known extruder either.